1. Field of the Invention (Technical Field)
The present invention relates to ion mirrors for mass spectrometry.
2. Background Art
In the earliest time-of-flight (TOF) mass spectrometers, ions were extracted from a source by a single linear extraction field to a field-free region. The arrival times of ions that traversed this region varied as a function of their m/z (mass/charge) ratios.
Two articles by Wiley and McLaren (Wiley, W. C.; McLaren, I. H. Rev. Sci. Instrum., 26, 1150 (1955) and Wiley, W. C. Science, 124, 817 (1956)) disclose that the space focus plane could be moved to the detector plane with a two-field extraction. Wiley and McLaren also combined this with time-lag extraction. Time-lag extraction transformed the ion thermal energy distribution into a spatial distribution that was subsequently corrected by space focusing at the detector. The disadvantage of the time-lag extraction is its mass dependence, which prevents simultaneous focusing over the whole m/z range.
An ion mirror introduced by Karataev et al. (Karaev, V. I; Mamyrin, B. A.; Shmikk, D. V.; A. Sov. Phys. Tech. Phys., 16, 1173 (1972)) solved the focusing problem reported by Wiley and McLaren. To solve the problem, a potential hill in the ion mirror was introduced, which produced a longer flight path for more energetic ions. Thus, due to the potential hill, two ions with the same m/z value but different kinetic energies spend different amount of time in the ion mirror. For example, an ion with higher kinetic energy spends less time in the field free region but penetrates deeper into the ion mirror, while an ion with lower kinetic energies spends more time in the field free region but penetrates the ion mirror less deeply. Thus, the ion mirror compensates for much of the difference in ion kinetic energies.
However, the ion mirror of Karaev et al. could not correct for initial kinetic energy distribution and/or spatial distribution of ions in the ion source at the same time. Essentially, the turn-around time of ions with random thermal motion in the source cannot be eliminated at the time of extraction; therefore, the turnaround time eventually limits the achievable resolving power unless random ion motion is avoided.
To effectively minimize the initial kinetic energy distribution along the time-of-flight (TOF) axis of an ion, orthogonal acceleration was introduced, referred to herein as xe2x80x9cTOF-oa.xe2x80x9d Theoretically, when TOF-oa is combined with a mirror that has an optimum field shape, a high-resolution mass spectrometer should be achieved.
In 1989, Dawson and Guilhaus built the first TOF-oa instrument for improving resolving power and duty cycle with an electron impact (El) ion source (Dawson, J. H. J.; Guilhaus, M., Rapid Commun. Mass Spectrom., 3, 155 (1989) and Dawson, J. H. J.; Guihaus, M. Australian Provisional Patent P16079, 1987; Int. Patent Appl. PCT/AU88/00498, 1988) and U.S. Pat. No. 5,117,107. According to the Dawson and Guilhaus instrument, ions are collimated by an electrostatic lens system and injected into an orthogonal extraction region. As a result, in a linear TOF instrument, the ion extraction and acceleration fields provide space focusing at the detector. The Dawson and Guilhaus instrumented reportedly achieved a resolution of 2000 at full width at half maximum (FWHM) of a spectral peak.
Dodonov et al. (Dodonov, A. F.; Chernushevich, I. V.; Laiko, V. V., International Mass Spectrometry Conference, Amsterdam, August 1991; Extended Abstracts, p153 and Dodonov, A. F.; Chernushevich, I. V.; Laiko, V. V. in Time-of-Flight Mass Spectrometry; Cotter, R. J. Ed.; ACS Symposium Series 549; American Chemical Society, Washington, DC, 1994. pp108-23) developed an orthogonal acceleration instrument that coupled electrospray ionization (ESI) and a dual-stage ion mirror mass analyzer with a resolution of about 1000 (FWHM).
Verentchikov et al. (Verentchikov, A. N.; Ens, W.; Standing, K. G., Anal. Chem., 66, 126 (1994)). reported an orthogonal acceleration instrument with a resolution of about 5000 (FWHM) by using a single-stage ion mirror. An improvement of this instrument reportedly achieved a resolution between 7000 and 10000 (FWHM) (Krutchinsky, A. N.; Chernushevich, I. V; Spicer, V. L.; Ens W.; Standing, K. G., J. Amer. Soc. Mass Spectrom., 9, 569 (1998)).
To date, ion mirrors have been a key element in providing improved resolution over the entire m/z range. In general, ion mirrors can be divided into two groups, linear and non-linear, according to the distribution of the electric field within the mirror. Linear ion mirrors are referred to as staged ion mirrors. Staged ion mirrors may have one or more stages, each stage having a linear electric field. In contrast, a non-linear ion mirror has an electric field contour that is curved along the mirror axis, particularly, in an ion turn-around region. Researchers have demonstrated that non-linear ion mirrors can achieve higher resolution than can linear ion mirrors (Cornish, T. J. and Cotter, R. J., J. Rapid Commun. Mass Spectrom., 8, 781-785 (1994)). Depending on the system, an xe2x80x9cidealxe2x80x9d non-linear ion mirror should exist. An ideal non-linear ion mirror preferably has an electric field with the theoretically optimum contour along the mirror axis and an absolutely homogeneous field in the off-axis directions. Inhomogeneity in the off-axis, or radial, directions results in ion dispersion away from the beam center and inequity in ion flight time across the useful beam diameter. Therefore, an ion mirror with a large off-axis homogeneous region near the beam center is desirable, in turn, an enlarged, useable beam center region results.
An xe2x80x9cidealxe2x80x9d ion mirror should achieve infinite order focusing of kinetic energy as reported by Rockwood, A L., Proceedings of the 34th ASMS Conference on Mass Spectrometry and Allied Topics; Cincinnati, Ohio, June 8-13, P173 (1986). The voltage in the electric field of an xe2x80x9cidealxe2x80x9d ion mirror follows the parabolic equation U=ax2 where a is a constant and x is the depth in the ion mirror along the axial direction. Unfortunately, such a parabolic field ion mirror is difficult to implement and has the disadvantage of having no field-free flight path.
To date, ion mirrors have primarily used two different configurations to create a non-linear electric field. One reported configuration uses stacks of many ring-like diaphragm elements (U.S. Pat. No. 4,625,112, entitled xe2x80x9cTime of flight mass spectrometer,xe2x80x9d to Yoshida, issued Nov. 25, 1986; U.S. Pat. No. 5,464,985, entitled xe2x80x9cNon-linear field reflection,xe2x80x9d to Cornish and Cotter, issued Nov. 7, 1995; U.S. Pat. No. 5,017,780, entitled xe2x80x9cIon reflector,xe2x80x9d to Kutscher et al., issued May 21, 1991) while the other configuration uses simple geometric shapes (Cornish, T. J; Cotter. R. J., J. Anal. Chem., 69, 4615 (1997); U.S. Pat. No. 5,814,813 entitled xe2x80x9cEnd cap reflection for a time-of-flight mass spectrometer and method of using the same,xe2x80x9d to Cotter et al, issued Sep. 29, 1998; U.S. Pat. No. 5,077,472, entitled xe2x80x9cIon mirror for a time-of-flight mass spectrometer,xe2x80x9d to Davis, issued Dec. 31, 1991).
Disadvantages of the stacks of ring-like diaphragm configuration are the non-homogeneity of the electric field in the off-axis directions and the number of conductive elements required. Each additional element adds critical spatial and voltage control requirements. Although the reported configurations that use simple geometric shapes are easier to implement for non-linear electric fields, off-axis homogeneity has, to date, limited the achievable resolution. Therefore, a need exists for an ion mirror that is not as limited by off-axis inhomogeneity or the requirements inherent in the use of a large number of elements.
U.S. Pat. No. 5,017,780, entitled xe2x80x9cIon reflector,xe2x80x9d to Kutscher et al., issued May 21, 1991, discloses an ion mirror with at least one special element of conical construction and many ring-like diaphragms. The implementation is difficult, in part, because all the conductive elements require distinct voltages and tight focusing of the ion beam close to the mirror axis, since their equipotential lines are not parallel and result in divergence of the ion trajectories for off-axis ions.
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
In a preferred embodiment, the present invention comprises an apparatus for affecting charged particles comprising at least two tube-shaped, electrically conductive elements arranged along a common axis wherein each of the at least two elements comprises a finite length; and at least one voltage source for providing a voltage to at each of the at least two elements wherein the provided voltage produces an electrical field comprising field lines perpendicular to the common axis for affecting charged particles travelling substantially parallel to the common axis. Preferably, only two or three elements are used to simplify the apparatus while still maintaining adequate operational characteristics. In a preferred embodiment, the elements are spaced along the common axis such that a gap exists between the elements. Of course, alternative embodiments wherein elements overlap, yet do not conductively touch, are also within the scope of the present invention. In addition, elements comprising more than one axis, for example, elements comprising two axes, are within the scope of the present invention, of course, the electrical field lines should be perpendicular to each of the axes.
In a preferred embodiment, each element comprises at least one cross-section normal to the common axis wherein the at least one cross-section comprises a shape selected from the group consisting of circular, ellipsoidal, oval, and polygonal shapes. Of course, an element optionally comprises other shapes; however, circular, ellipsoidal, oval and/or polygonal shapes are preferred. In another preferred embodiment, cross-sectional area varies along the common axis. In such an embodiment, the cross-sectional area increase and/or decreases along the common axis. In a preferred embodiment, at least one element comprises a constant cross-section and cross-sectional area.
In a preferred embodiment, at least one electrical field comprises a non-linear electrical field along the common axis. In such an embodiment, the non-linearity optionally comprises a mathematically calculated and/or experimentally derived non-linearity that is useful for affecting charged particles for a particular purpose. For example, in ion mirror embodiments of the present invention, non-linearity serves to provide at least first order focusing, and preferably at least second order focusing.
According to a preferred embodiment, the present invention comprises at least one grid wherein the at least one grid is optionally integral with at least one of the at least two elements. An element optionally comprises a grid at any point along its axis. Likewise, in a preferred embodiment, the present invention comprises at least one plate wherein the at least one plate is optionally integral with at least one of the at least two elements. An element optionally comprises a plate at any point along its axis. In a preferred embodiment, a plate defines at least one aperture, and preferably a single aperture.
In a preferred embodiment, the apparatus comprises a charged particle mirror wherein charged particles enter the apparatus substantially parallel to a common axis, reverse direction and exit the apparatus substantially parallel to the common axis. In a preferred embodiment, the mirror provides for at least first order focusing of charged particles and preferably at least second order focusing of charged particles.
The present invention is not limited to charged particle mirrors, for example, the apparatus optionally comprises a charged particle lens. As disclosed herein, the term ion is used in describing several embodiments; it is understood to one of ordinary skill in the art of physics and/or chemistry that an ion is a charged particle and that the ion embodiments are useful for charged particles in general. Charged particles include, but are not limited to, ions and electrons.
In a preferred embodiment, the inventive apparatus comprises a charged particle zoom lens comprising at least one element movable along said common axis. In such an embodiment, the lens comprises a variable focal length.
In a preferred embodiment, the present invention comprises a single front element for use with a device for affecting charged particles wherein the single front element comprises an increasing cross-sectional area from front to rear. In such an embodiment, this front element further comprises a front plate defining an aperture. According to the present invention, such an embodiment is useful for replacing a grid, for example, a front grid. Of course, embodiments of the present invention described herein optionally comprise a front element comprising an increasing cross-sectional area from front to rear. Furthermore, the increase in cross-sectional area is optionally linear and/or non-linear and/or with changing cross-section shape in addition to dimensions.
According to a preferred embodiment, the apparatus comprises a mirror and/or a lens wherein charged particles enter and exit along a common axis. Such embodiments include apparatus wherein charged particles enter at an angle and exit at another angle and/or the same angle to the common axis. In a preferred embodiment, such angles comprise angles of less than or equal to approximately 15 degrees. Of course, embodiments comprising larger angles are within the scope of the present invention. However, for example, in the case of a mirror, care must be taken that the field is relatively homogeneous in the radial direction encompassed by the angle about the axis, i.e., it is best to use angles that maintain the charged particles within a radially homogenous field region. Angles encompassed by the present invention correspond to angles used in charged particle devices known to one of ordinary skill in the art, for example, mass spectrometer devices. In general, mass spectrometers use angles that are substantially parallel to an ion mirror axis. Radially homogenous field refers to a field that is substantially the same on the axis as in a radial position off that axial point. Experiments presented below demonstrate the balance between radial field homogeneity, ion beam size and resolution in a mass spectrometer.
In a preferred embodiment, the elements of the apparatus comprise an orthogonal arrangement about the common axis. In such an embodiment, a gap of uniform widths is preferably formed between adjacent elements.
In a preferred embodiment, the present invention comprises an ion mirror for mass spectroscopy comprising at least two tube-shaped, electrically conductive elements arranged along a common axis wherein each of the at least two elements comprises a finite length; and at least one voltage source for providing a voltage at each of the at least two elements wherein the provided voltage produces an electrical field comprising field lines perpendicular to the common axis for reflecting ions travelling substantially parallel to the common axis. In a preferred embodiment, the ion mirror provides for second order focusing of an ion beam.
A primary object of the present invention is to improve resolution of mass spectrometers.
A primary advantage of the present invention is the production of off-axis field homogeneity.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.